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Advantages of using differential pair OTA instead of pseudo differential structure

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safwatonline

safwatonline

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is there any obvious benefit of using a differential OTA over using pseudo differential structure (two single ended OTAs) as i see that the only issues are PSR and CMR? are they worth it, given that the complexity of the differential OTA (like needing CMFB)? i.e. cannot we like match the two SE OTAs enough not to notice the difference?
 

Re: Differential pair

That is a good point.

Supply sensitivity which is the application of common mode rejection in most OTA's is also defined by the output stage. So if the input stage losses e.g. 6dB voltage range in comparison to a pseudo-diff-stage the overall, to be clear, relative rejection could be worse. The focus on input rejection is historic because most opamps have high gain so input stage CMRR matters. But OTA is a different story.

To give you a small hint; there three CMRR effects!

1. Simple ratio of Diff-to-Diff versus CMD-to-CMD gain
2. Mismatch based CMRR
3. Dynamic CMRR, if the signal swing is big CMD-signal could couple in or mixed
 

    safwatonline

    safwatonline

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Differential pair

Pseudo diff pair could give higher swing for us as it does not involve the tail transistor and we save one over drive voltage there!

But i think we need CMFB even in pseudo diff pair.. I used CMFB even in the pseudo diff pair in my 2 stage OTA where in first stage is a fully differential and 2nd stage is pseudo diff..

Anyways CMR is an important factor for most of the applications and hence if we are to use single stage, then its better to go for fully diff structure unless and until you are sure that you dont have common mode noise in your signal..
 

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